Within the fungal genus Pythium are plant pathogenic species which can cause significant losses in vegetable production. The value of vegetable crops grown in the state of Florida totaled over $1.79 billion for the 1991-92 crop season with much of the cabbage, cucumber, pepper, and tomato crops planted as transplants. Tomato alone accounts for $735 million in this production total and over 80% of the crop is transplanted. The use of vegetable transplants in commercial field production systems is important in many areas of the United States. In California, all of the celery, fresh market tomato and pepper, and most of the cauliflower and broccoli are grown as transplant crops. In Monterey County alone, the value of the transplants grown in 1992 amounted to nearly $18 million and represented a final crop value of $176 million.
Plant pathogenic Pythium species can kill a plant at the seedling stage or can reduce crop yield by destroying the root system of a mature plant. While diseases in the seedling stage are often controlled by fungicide application, the continued use of certain highly effective fingicides, e.g., metalaxyl, has faced regulatory uncertainty for use in vegetable transplant greenhouse production systems. In addition, continued use of a particular fungicide can result in the development of tolerance by the pathogen. Fumigants such as methyl bromide, which are routinely used on high cash-value crops, also face regulatory uncertainty. Thus, disease control (in particular damping-off) in the production greenhouses as well as in the field following transplanting are a major concern.
In addition to the plant pathogenic species of Pythium, some members of this genus exist strictly as soil saprophytes (Van der Plaats-Niterink, 1981, Monograph of the genus Pythium. Studies in Mycology No. 21. Centraalbureau Voor Schimmelcultures, Baarn, The Netherlands), and several have been identified that are pathogenic on mammals (de Cock et al., 1987, J. Clin. Micro. 25:344-349), fish (Van der Plaats-Niterink, 1981), and insects (Saunders et al, 1988, J. Invert. Path. 52:360-363). One species, Pythium oligandrum Drechsler, is not pathogenic on plants and is effective in protecting plants from attack by pathogenic species.
An alternative to the use of pesticides for controlling phytopathogenic Pythium spp. is the use of biological control agents for vegetable transplants, a large and expanding industry in which disease protection is needed in the greenhouse as well as in the field after transplanting.
U.S. Pat. No. 4,574,083 to Baker and Lifshitz describes Pythium nunn, which is not pathogenic to plants and can protect seedlings from damping-off in greenhouse evaluations. However, P. nunn generally grows slower in culture medium than P. oligandrum but, in contrast to P. oligandrum, can colonize organic substrates in the soil that have already been colonized by other fungi. Pythium nunn also can be difficult to sporulate; sporulation is an essential characteristic for inoculum production. In the soil, P. nunn also behaves differently than P. oligandrum in that, as a primary colonizing saprophyte, it is not as aggressive.
There are a number of studies examining the effect of seed treatment with oospores of P. oligandrum on reducing subsequent levels of disease, most of which have been conducted in the greenhouse. Deacon (1976; Trans. Br. Mycol. Soc. 66:383-391) described the ability of mycelial seed coatings on wheat to significantly reduce the disease incidence over untreated seeds.
Vesely (1977; Phytopath Z. 90:113-115; 1979) observed that application of oospores to sugarbeet seed reduced damping-off incidence to a similar level as thiram treatment (see also Schippers, B. and W. Gams, eds. Academic Press, Soil-Borne Plant Pathogens). In U.S. Pat. No. 4,259,317, Vesely et al describe the application of Pythium oligandrum, or "Polygandron." However, the '317 patent describes a particular isolate P. oligandrum for protecting against damping-off by applying a preparation to sugarbeet seed. There is no description or suggestion in the '317 patent regarding the use of Polygandron on vegetable transplants.
A co-worker and I observed a similar response for disease control when using isolates of P. oligandrum recovered from suppressive soils pelleted onto sugarbeet seeds. See Martin and Hancock (1987; Phytopathology 77:1013-1020). While the emerging radicle was colonized by the biocontrol agent and protected from plant infection, this colonization was limited to association with the seed coat and suggested that the fungus was not rhizosphere competent. Additional greenhouse trials on cress (Al-Hamdani et al, 1983, Plant Pathology 32:449-454; McQuilken et al., 1990 Plant Pathology 39:452-462, 1992 J. Phytopathol 135:125-134); sugarbeet and carrot (Lutchmeah and Cooke, 1985, Plant Pathology 34:528-531); and sugarbeet (Walther and Gindrat, 1987, Journal of Phytopathology 119:167-174) have indicated that seed treatment with P. oligandrum can control other phytopathogenic Pythium spp., Phoma betae, and Mycocentrospora acerina.
A biological agent that could protect seedlings from disease in the production facility and after transplanting into the field would meet an existing need and provide an advantageous method for controlling disease, e.g., damping off, in agricultural industries.